The invention relates to a vehicle hydraulic brake system
A vehicle hydraulic brake system of this kind has been disclosed by DE 195 01 760 A1. The known vehicle brake system has a master cylinder, a number of wheel brake cylinders, and a hydraulic pump with which brake fluid can be delivered from the master cylinder to the wheel brake cylinders. The hydraulic pump is used for hydraulic brake boosting. It can likewise be used for antilock braking, traction control, and electronic stability programming, wherein in order to reduce pressure in the wheel brake cylinders by way of valves, the intake side of the hydraulic pump can be connected to the wheel brake cylinders and the pressure side of the hydraulic pump can be connected to the master cylinder. It is conventional per se to embody the hydraulic pumps in vehicle brake systems of this kind as dual piston pumps with pistons disposed opposite each other in a boxer arrangement, which are driven by a common cam disposed between the two pistons. The two pistons operate in anti-phase, i.e. while one of the two pistons is executing a delivery stroke, the other piston is executing a return stroke. The delivery stroke is the stroke in which the piston decreases the volume of a displacement chamber in a cylinder of the piston pump and thus displaces fluid from the piston pump. In the return stroke, the volume of the displacement chamber is increased again; this stroke is also often called the intake stroke. Due to their oscillating operation, piston pumps have an oscillating intake volume flow and cause pressure pulsations on the intake side, which have repercussions on the master cylinder and produce an unpleasant sensation in a foot brake pedal and generate clearly audible noise. Both of these are undesirable, particularly if the hydraulic pump is used for hydraulic brake boosting, i.e. is operated with each braking maneuver. However, in antilock braking, traction control, and electronic stability programming as well, which are only active in unusual driving situations, it is at least desirable to reduce the pressure pulsations. It is also known to embody the pistons of the piston pump as stepped pistons which have the advantage of aspirating brake fluid during both the delivery stroke and the return stroke. A stepped piston has the advantage over a simple piston of a more uniform intake volume flow with a reduced amplitude and a doubled frequency.
The vehicle hydraulic brake system has a multi-piston pump with an even number of stepped pistons. The stepped pistons are driven in a phase-shifted but not anti-phase manner, i.e. are not driven with a phase angle of xcfx80. The stepped pistons of the vehicle brake system according to the invention therefore reach their dead centers one after another. The intake of the stepped pistons from the master cylinder therefore takes place in a time-delayed fashion, wherein the intake volume flows of the different stepped pistons partially overlap. This has the advantage that the entire intake volume flow of the multi-piston pump, i.e. the sum of the intake volume flows of all of the stepped pistons, is more uniform. The individual intake volume flows overlap one another, and the total intake volume flow fluctuates with a reduced amplitude around an average. The magnitude of the pressure pulsations on the intake side of the piston pump, which have repercussions on the master cylinder, are reduced. It is therefore immaterial with regard to the intake side whether successive stepped pistons reach their top dead centers in succession and then reach their bottom dead centers in succession or whether in succeeding stepped pistons, a preceding stepped piston reaches the top dead center and a subsequent stepped piston reaches the bottom dead center and vice versa. In any case this is true if the intake volume flows in the delivery stroke and in the return stroke are approximately the same size. For the pressure side, an evening out of the volume flow occurs when, with a multi-piston pump that has an even number of four or more stepped pistons, the succeeding stepped pistons alternatingly reach the top dead center and the subsequent stepped pistons reach the bottom dead center.
The invention is provided in particular for a vehicle hydraulic brake systems with electrohydraulic brake boosting, i.e. in vehicle brake systems in which a brake pressure built up in the master cylinder is increased through the use of the multi-piston pumps, or for electrohydraulic vehicle brake systems in which the brake pressure is produced with the master cylinder only in the event of a malfunction but during normal operation, is not produced with the master cylinder, but only with the multi-piston pump since with these vehicle brake systems, the multi-piston pump is actuated with each braking operation and its repercussions on the master cylinder can be felt. However, the invention is also advantageous for other vehicle hydraulic brake systems with a hydraulic pump, for example for antilock braking, traction control, and/or electronic stability programming, even when the multi-piston pump in such vehicle brake systems is not operated during normal braking maneuvers without slippage in the vehicle wheels, and the advantages of the invention only become apparent once the antilock braking, traction control, or electronic stability programming is used.
In accordance with the invention, multi-piston pumps are also understood to mean several piston pumps that are independent of one another and are synchronized with each other, for example electronically or mechanically, for example by means of gears. Thus for example, two piston pumps that are synchronized with one another can be used for the two brake circuits of a dual-circuit vehicle brake system. Possible designs include, for example, star-shaped arrangements of the stepped pistons around a common cam, also a number of stars in succession, rows, V-shaped arrangements or boxer arrangements of stepped pistons with an eccentric shaft, camshaft, or crankshaft for driving the stepped pistons in a synchronous, phase-shifted manner.
Advantageous embodiments and improvements of the invention disclosed herein are the subject of the invention.
The stepped pistons of the multi-piston have a phase shift of approximately xcfx80 divided by the number of stepped pistons or a multiple of this value, but do not have a phase shift of xcfx80 because this would be anti-phase. One stepped piston would always be executing the delivery stroke and at the same time, a piston operating in anti-phase to this would be executing the return stroke. The two anti-phase pistons would aspirate simultaneously since they aspirate both during the delivery stroke and the return stroke. This would not lead to a reduction of the pulsation by means of the overlapping of the intake volume flows, but would on the contrary lead to an addition and therefore to an intensification of the pulsation. The phase shift of xcfx80 divided by the number of stepped pistons produces a uniform chronological distribution of the intake volume flows of the stepped pistons. Deviations from this phase shift can be suitable, e.g. for the sake of the space-saving accommodation of the stepped pistons in a pump housing.
The vehicle brake system according to the invention can be embodied as a single-circuit brake system with a multi-piston pump that has at least two stepped pistons which act on a brake circuit.
It is also possible to embody the vehicle brake system according to the invention as a multi-circuit, for example dual-circuit, brake system which has at least one stepped piston for each brake circuit. The pressure pulsations occurring on the intake side of the multi-piston pump in each brake circuit are compensated for in a common master cylinder. Preferably, an at least partial pressure compensation takes place between the brake circuits for example by means of a master cylinder which has a floating piston. claims.
The provision is made that the stepped pistons have an approximately equal intake volume in the delivery stroke and in the return stroke, i.e. on an end that is tapered due to the piston step, the stepped pistons have approximately half the cross-sectional area as on their other end.